The present invention relates to an apparatus for recording information in an optical information recording medium and/or reproducing information from an optical information recording medium, respectively by utilizing holography.
An optical disc having a record density of about 50 GB is now available even as a consumer product, using a blue-violet semiconductor laser and the Blu-ray Disc (BD) specifications. It is desired for the future that even an optical disk has a large capacity of 100 GB to 1 TB like a hard disc drive (HDD).
However, in order to realize such an ultra high density by an optical disk, high density techniques of a new type are required which are different from those of short wavelength and high NA objective lens.
While studies on next generation storage techniques are made, hologram recording techniques of recording digital information by utilizing holography have drawn attention. According to the hologram record techniques, signal light having information on page data two-dimensionally modulated with a spatial light modulator and reference light are superposed in a recording medium, and a generated interference pattern forms a refractive index modulation in the recording medium to record the information in the recording medium.
In reproducing information, as the reference light used for recording is applied to the recording medium, a hologram recorded in the recording medium operates like a diffraction grating to form diffraction light. This diffraction light is reproduced as the same light containing the recorded signal light and phase information.
The reproduced signal light is two-dimensionally detected at high speed with a photodetector such as a CMOS and a CCD. With the hologram recording techniques, two-dimensional information can be recorded in a recording medium at a time by one hologram, and the recorded information can be reproduced. Since a plurality of page data sets can be written in a stacked manner in an area of the recording medium, a large amount of information can be recorded/reproduced at high speed.
When an apparatus or an information recording medium utilizing such hologram recording techniques is manufactured, it is necessary to use signal quality evaluating means in order to judge whether a recording performance or reproducing performance of the apparatus or the performance of the information recording medium is sufficient or not.
As indices to be used by a signal quality evaluation method, there are an SNR (Signal to Noise Ratio) and a BER (Bit Error Rate).
The definition described in “System and Material for Holographic Memory” under supervision of Tsutomu SHIMURA is generally applied to SNR of hologram recording techniques. SNR is represented by:
      S    ⁢                  ⁢    N    ⁢                  ⁢    R    =                    μ        on            -              μ        off                                      σ          on          2                +                  σ          off          2                    where σon and σoff, and μon and μoff are standard deviations and average values, respectively of the luminance value distribution of on-pixels and off-pixels of reproduced page data.
BER is an evaluation index representative of a ratio of a bit error of a reproduced signal to recorded data. A method of calculating BER from on-pixels and off-pixels is described, for example, in JP-A-2008-97688.